The problem has been very irregular. Sometimes a change seems to work then several hours later, the problem returns. USB adapter speed does not seem to be the answer as it's the same on a Win7 desktop with USB2 ports and a Win10 laptop with both USB2 and USB3. Hubs and active extensions have no effect. Also, the same sonar sketch uploads, runs, and prints consistently on a T3.2 and T3.6. Those two boards have always worked correctly. The problem occurs on both T4.0 and T4.1 boards. One clue that shows up occasionally is an error message that "Windows reports the port is ready when it's not ready". The next time I see it I'll copy and paste the entire message on this thread.